Strain measuring system



l' Oct. 24, 1944. T. E. BRowNE, JR 2,361,173

STRAIN MEASURING SYSTEM Filed March 24, 194s ATTORNEY Patented Oct. 24, 1944 STRAIN MEAsUmNG SYSTEM Thomas E. Browne,

Jr., Wilkinsburg, Pa., as

signor to Westinghouse Electric & Manufacturing Company,

East Pittsburgh, Pa., a corporation of Pennsylvania l Application March 24, 1943, Serial No. 480,325 claims. (c1. 177-351) 4 vThe present invention relates generally to strain measuring systems and more particularly to systems of the character employing electromagnetic devices for detecting the stress or deflection to be measured.

Electromagnetic devices or reactors have been widely used in the past for detecting the deflections of a member subjected to a stress. However, because of the non-linear response of such devices when used singly in a strain measuringl system and other non-linear characteristics of the measuring system associated with the reactor, the practice has been to use two or more reactors with their coils arranged in the legs of a bridge circuit so that the non-linearities of corresponding pairs vof the reactors will cancel.

In some applications of stress or strain measurement, it has been found that the best mechanical arrangements have made it undesirable or even impossible to employ the usual plurality of reactors arranged in the conventional bridge circuit and in order to obtain the desired proportional stress jor strain indication with a single reactor detecting the stress or strain, some arrangement whereby the above-mentioned non-linearities may be corrected must be provided. Such correction means may be conveniently provided in the measuring system.

In order to understand the present invention more completely, an understanding of the speciflc characteristics which must be corrected should be had.

Electromagnetic devices or reactors of the character referred to usually comprise a generally U-shaped or E-shaped magnetic core member, a portion of which is disposed within a magnetizing coil. An armature member bridging the extremities of the core member in close proximity thereto is provided to vary the reluctance of the magnetic circuit. Thus the reactance or impedance of the `coil linking the magnetic circuit is caused to be varied by armature displacement. However, such an impedance change is not linearly proportional to the armature displacement primarily because the leakage flux at the airgaps, formed between the core extremities and the armature, increases with an increasing airgap and as a result variations in rate of response to the armature movementl are introduced into the metering system adapted to measure the varying output of the coil.

If the impedance of the metering or recording device in circuit with the magnetizling coil were zero and could-be neglected, the currentilowing in the metering system would vary in a linear the admittance of the magnetizing coil. Thenonlinearities thus existing in a system employing but a single reactor as the deflection detecting element are additive and the resulting indication by the metering device does not have simple proportionality to the armature displacement, thus requiring that the readings of a linear metering device, such as an oscillograph galvanometer, be interpreted by means of a calibration curve rather than by a simple proportionality constant.

Accordingly, it is a principal object of the present invention to provide a stress or deflection measuring system employing but a single electromagnetic device or reactor as the deflection detecting element which linearly indicates the deflections to be measured over the whole of a predetermined working range of deflections.

An ancillary object of the present invention isto provide a deilection measuring system of the character referred to in which a current which increases with a decrease in the airgap of the reactor is produced.

Another object of the present invention is to provide a. stress or deiiection measuring system of the character referred to in which the nonlinear response of the reactor to changes in its airgaps and thenon-linear variation of current flowing in theI measuring system with respect toV changes in coil impedance of the reactor are made subtractive and thus, in effect, cancel each other.

A specific object of the present invention is to provide a stress or deiiection measuring system of the character referred to in Which the coil of the reactor is shunted by a capacitor of such value that the parallel impedance has a leading powerfactor over the whole of the working range of airgaps of the electro-magnetic device and to further provide a value of impedance in the metering or recording device such that the effect of the non-linear response of the reactor and the effect of lthe non-linear relation of current in the measuring system to changes in coil impedance are made subtractive and thus in effect cancel each other. I

Other objects and advantages willbecome more lapparent from a study of the following specimanner with the reciprocal of the impedance,

or admittance, of the coil. Since, however, the impedance of the metering device is of some finite value, the current iiowing in the metering system will no longer have a linear relation with Fig. 3 graphically illustrates the resultant susceptance of the paralleled coil and condenser elements of F18. 1.

Eig. 4 graphically illustrates the non-linear relation of the current in the metering system t0 the susceptance of the paralleled coil and conchange.

ance' of thecurrent responsive element of the metering or recording instrument.

denser combination for given values of imped,

increasing pressures tend to close the airgaps l,

' the length of the airgaps Lwill be decreased and Fig. graphically illustrates the linear rela- A tionship of current in the metering circuit to changes in airgap length of the electromagnetic device with a properly selected value of impedance of the current responsive element of the metering or recordng instrument.

Fig. 6 is a practical embodiment of the fundamental circuit of Fig. 1, and

Fig. 'I diagrammatically illustrates a preferred form of a deflection measuring system embodying'the principles of the present invention.

The basic principles of the present invention may best be understood by referring to the simple series circuit of Fig. 1. 'I'he specific arrangement shown is adapted to measure fluid pressures and comprises a chamber l having one end thereof/ sealed by a flexible diaphragm 2. This chamber is adapted to be fixed such that the open endV thereof is in communication with a container (not shown), the fluid pressure in which is to be measured. Thus deflections of the diaphragm 2 will indicate the pressure within the container. An electromagnetic device or reactor generally indicated by numeral 3 has the armature element 4 thereof positioned to move in accordance with deflection of the diaphragm 2. Thus any change of applied pressure on the diaphragm will'vary the length L of the airgaps 5 formed between the f extremities of the core member of the reactor 3 and the armature.

As a step towards compensating for the nonlinear response of the reactor 3, a capacitor 6 is arranged in parallel circuit relation with the coil element 1. The value of this capacitor is such that` a leading power factor is obtained over the Whole of the working range of airgaps 5Y whereby resulting current values areproduced as vectorially indicated in Fig. 2.

In Figs. 1 and 2, Ir. denotes the current flowing through the coil 'i of the reactor 3, Ic is the current flowing through the capacitor 6, Ex the voltage across the paralleled coil 1 and capacitor 6. I the current output of the parallel circuit and Z' the overall impedance of a metering circuit 8 ivrcluding any suitable metering or recording de- .Assuming a deflection of the diaphragm due to an increasing pressure and a corresponding decrease of the airgaps 5, an increase in the impedance of the cil 1 follows which limits the current Ii. flowing therethrough from some larger value 11.1, on the vector diagram of Fig. 2 to some smallervalue 11.2. In view of the leadingcurrent Ic of the capacitor 6, a variation in the current I is obtained from I1 to In. Thus it will be noted that it Ir'.1 and Ir.2 represent respectively the maximum and minimum currents through the coil 1 for the 'maximum and minimum airgaps, a'current output of the paralleled may be expressed AL=Lo-L, where Lo is the value of L at zero pressure. AL is, therefore, the amount by which the gap is decreased. The lagging susceptance br. of the reactor 3 is plotted below the abscssa and the leading susceptance bc of the capacitor 6 is plotted above the abscissa. The algebraic addition of these two curves since their effects are in opposition gives the plot b1 and similar to the vector diagram of Fig, 2 indicates how the addition of the overbalancing capacitor 6 changes the over-al1 susceptance from one decreasing numerically to one increasing numerically with increasing AL which corresponds to a decreasing airgap. Inother words, the current output of the paralleled coil and condenser combination will increase with decreasing susceptance of the coil'. In each case, however, it will be noted that the decrease in flux leakage with a decreasing airgap increases the slope of the curves.

Figure 4 graphically illustrates the means for correcting this change in slope. Here the current I in the metering circuit is plotted against the total susceptance, b1. Referring to Fig. 1, with'a constant voltage E, a substantially linear rela-` Under such condition and neglecting the resistcircuit is obtained which leads the voltage Ex f and which increases in value with increasing nuid pressure. Thus it will be seen that the use of the overbalancing capacitor C causes a revcrsal of sign of the rate of change of I with respect to In and hence with respect to the airgap The change in the current I, however, is still non-linear with respect to the change in airgap. This may best be understood upon an inance of the parallel circuit the curve of Fig. 3 may substantially indicate the variation in output current I with changes in AL. This linear relationship is indicated by the dotted line Z=0 in Fig. 4. Since the impedance relationship mentioned may not practically be obtained because Z., is of some measurable value with respect t0 the overall impedance of the parallel circuit, the actual relationship of the current I, to susceptance b1 is of the nature indicated by the solid curve, Z finite, for some predetermined value of Z. Thus, increasing the value of the impedance 4Z of the metering circuit l increases the departure from a linear relation betweenthe current I and the susceptance bi, l

The veiect upon the current I of the two sources of non-linearity, namely, the leakage ux of the reactor 3 and the impedance of the metering circuit-I which normally are additive, are now Y made subtractive by means of the overbalancing capacitor 6. By proper adjustment of the impedance value Z it willv now be seen that the noninear effects may be canceled to produce a current I which willv vary linearly with AL as shown in Fig. 5. Y l lIn practicing the present invention, various modincationsfof the fundamental circuit of Fig. 1 are, of course, obtainable. Fig. 6 illustrates a strain measuring system which is specifically adapted to indicate the instantaneous pressures developed -within thearcing structure of an oil circuit breaker or the like. The primary I of a transformer 'le is connected to a suitable source of alternating potential. A secondary winding Il of the transformer Il has connectedin series therewith'the parallel circuit comprising the cou' 1 of the reactor l and the capacitor l. v"'The output of this circuit as controlled by movements of the armature 4 is supplied to a bridge rectifier `i2 and thence through conductors I3^and I4 to a suitable nlter l5. The rectifier and filter are necessary to provide the desired continuous cur- V,lasciava rent to the current responsive element 8' comprising the detecting element of an oscillograph generally indicated at I6. Another secondary winding I1 of the transformer I0 supplies a second bridge rectiiler I8, the rectified current output of which is limited by an adjustable resistor or potentiometer I9 and supplied to the conductors I3 and I4 in opposition to the rectied current output of the bridge rectifier I2. The purpose of this second circuit is three-fold. Since its current is in opposition to that in the circuit including the paralleled coil 'I andcondenser 8, it makes possible the 'adjustment to zero by means of the resistor slider 20. of the current supplied to the oscillograph element 8' while there is still an arbitrarily determined current flowing in the main circuit branches. Thus the response line of Figure 5 can be changed to the dotted line through the origin, which dcttedline shows not only la linear relation .but also a linearly proportional relation between the output current I and the airgap change AL. In addition to providing zero indication of the oscillograph forI zero pressure, this permits a higher base current to be used through the reactor 3, thus giving more available current for the osciilograph for the same proportional change in reactor current. It ilnally'tends to improve the linearity of the calibration curve near zero pressure by avoiding operation of the bridge rectiflers near zero current where their apparent resistance becomes very high. VShould the impedance of the metering circuit 8 including the current responsive element 8 of the .oscillograph I6 and its associated filter IEJ/be insumcient to produce a linear current as previouslyI explained in connection with Fig. l, a resistor 2i of suitable value may be connected in series 'circuit relation therewith to produce an over-all impedance Z of the necessary amount. In addition it may be desirable in such a circuit to provide a portion of the total impedance Z in series on the A. C. side of the rectifier I2 as, indicated at 2Ia.-

The preferred embodiment of the present invention is illustrated in Fig. 7. In this application, the secondary winding 22 of the transformer I0 is center-tapped and each half thereof comprises one leg of a bridge circuit. The other two legs of the bridge circuit are formed by the primary windings 23 and 24 of the isolation transformers 25 and 28. The output terminals of the bridge circuit thus formed have connected thereacross the input terminals of a bridge rectifier 21 and a capacitor 28 in series therewith. The output terminals of the bridge rectifier supply a suitable lter 29 through conductors 30 and 3| and-the filtered output is conducted through the current responsive element 8' of the oscillograph I8. In the present instance, itor 28 are provided ineffective series circuit relation with the filter 29 and the oscillograph element 8 to produce the over-all .impedance Z of the current measuring circuit 8 which is necessary to obtain the previously explained linear current. Further refinement in linearity oi response can be obtained by suitably proportioningthc resistive and capacitive components of Z. A moderate D. C. bias is still provided from a separate source,

` such as a battery, by means of conductors 30' and 3|', to eliminate near zero current nonlinearity-due to variable resistance of the bridge rectifier 21 and to imperfect matching of the bridge circuit elements. Adjustments-"for zero overall output for zero measured pressure can be made by adjusting this bias as well as by adjustu both resistor 2| and capacing the bridge circuit reactor 'to be mentionedA Vfor convenience and serve to isolate the circuit,

make possible more convenient capacitance values and to minimize the effect of line drop in the line connecting the pressure-responsive reactor 3 to the other components of the parallel circuit. The paralleled coil l '6 are supplied by the secondary winding 32 of the transformer 25, and a similar circuit comprising a variable inductance` coil 33 shunted by a capacitor 34 is supplied by the secondary winding 35 of the transformer 2G. The series condensers 36 and 31 appearing on the secondary sides of the coupling transformers 25 and 26 have been provided to cancel the leakage reactance of the transformers, thus keeping their effective voltage ratio practically equal to their turn ratio. Balancing of the bridge circuit is accomplished by properly adjusting the variable inductor 33 until the unbalanced current I owing from the output tervminals of the bridge circuitis zero. In' view of the direct current bias which opposes ow of the current I, a normal highercirculating current `may be maintained in the bridge circuit. This,

of course, results in a proportionally greater output current I when an unbalance of the bridge circuit occurs. 'Ihe range'of current values of I flowing through the recter 21 may therefore be kept on the more at portion of the characteristie curve of the rectiiler with a resulting substantially linear rectified current output; The bridge circuit is unbalanced by a reduction in the airgaps 5 in accordance with increasing pressures which causes an increase of impedance of the parallel circuit and a corresponding change in the loading of the transformer 25. An unbalanced ow of leading current I increasing non- Y linearly with the impedance change, will, therefore, flow through the bridge rectifier 2'I the condenser 28, and lter 29 and which due to the eiect of the over-all impedance of the metering circuit 8 including thecurrent responsive element 8' the condenser 28, the lter 29 and the resistor 2|, will be caused to vary in a linear manner with the impedance change of the coil 'I to produce a graphic indication by means of the oscillograph generally indicated at I6 which m accurately indicative of the instantaneous pressure in a linearly proportioned manner.

The foregoing. disclosure and the showings made inthe drawing are merely illustrative of the principles vo1 this 'invention and are not to be considered in a limiting sense. The only limitations are to be determined from the scope of the appended claims.

I claim asmy invention:

` 1. A system for indicating the deilections of a member subject to a stress comprising, in combination, a variable reactor variable in response to deflections of said member, said reactor having a lagging current output which varies in a nonlinear manner with deflections of said member. means in circuit relation with said reactor for producing a circuit characteristic whereby a leading non-linear current is obtained, means for correcting said non-linearity whereby a leading current varying linearly with deflections of said member is obtained and means for measuring said current whereby the deflections of said member are indicated.

2. A system for indicating the deflections of of the reactor 3 and the capacitor tions of said member is obtained and means for -measuring said current whereby the denections of said member are indicated.

3. A system for indicating the deilections of a member subject to a stress comprising, in combination, a reactor including a coil the inductance of which is variable in response to deilections of said member, said coil having a lagging current output which varies in a non-linear manner with deilections of said member, a capacitor arranged in shunt circuit-relation with said coil adapted to change said lagging non-linear current to a leading non-linear current, means for correcting said non-:linearity whereby a leading current varying linearly with deflections of said member is obtained and means for measuring said current whereby the deections of said vmember are Aindicated.

4. A system for indicating member subject to a stress comprising, in co'mbination, a reactor including a coil in the inductance of which is variable in response to deiiections of said member, said coil having a lagging current output which varies in va non-linear manner with deflections of said member, a capacitor arranged in parallel circuit relation with said coil having a value of capacitive susceptance in excess of the maximum attainable inductive susceptance of said coil whereby a leading current is obtained which varies non-linearly with defiections of said member, means for correcting said non-linearity, whereby a leading current varying linearlyl with deflections of said member is obtained and means for measuring the current whereby the deflections of said member are indicated.

5. A systemV for indicating the' deflections of a member subject to a stress comprising, in combinatlon, a reactor having a coil inductively variable in response to deilections of said member, said coil having-5a lagging current. output which varies non-linearly with deilectlonsof said member, a capacitor arranged in parallel circuit rela--A tion with said coil having a value of capacitive susceptance in excess of the maximum attainable inductive susceptance of said coil lwhereby a.

leading current-is obtained which varies non-l linearly with deilections of saidmember, an additional impedance device in circuit relation with said coil of such characteristic as to have an opposite non-linear effect on said current of such amount that said current varies linearly with deections of said member and means for@ measuring `the current whereby the deflections of said member are indicated.

6. In a system employing but a single .reactor a member subject to a stress and having `Vla. lagging non-linear current output with respect to deflections of said member, means ,for correcting the non-linearity of said current comprisj ing, in combination, means in circuit relation with said coil for providing a circuit characteristic whereby a leading non-linear current is obtained, and means in circuit relation with said said member, means shunting said the deflections of a including a coil for detecting the deflections *of` coil of such character as to cause said current to vary linearly with deilections of said member.

'1. Apparatus of the character set forth in claim 6 in which said means for providing a cir- Y 5 cuit characteristic whereby a leading non-linear current is obtained is a capacitor disposed in parallel circuit relation with said coil.

8. Apparatus of the character set forth in claim 6 in which said last mentioned means comprises l0 a circuit for measuring said current.

9. A system for measuring the deilections of a member subject to a stress comprising, in combination, a reactor including a coil the inductance of which is variable in response to deflections of said member, a source of alternating potential for energizing said coil, said .coil havin'g a lagging current output which varies nonlinearly with deilections of said member, means in parallel circuit relation with said coil for providing a circuit characteristic such that a leading non-linear current is obtained, circuit means including a full7wave rectifier for measuring said current, said circuit means having a suitable l value of impedance to correct the non-linearity of said leading current, and biasing means for supplying a direct current bias to the output of -said rectiiler thus fixing the minimum value of current therein whereby near-zero current nonlinear characteristics of said rectler are elimimted.

l0. A system for measuring the deilections of a member subject to a stress comprising, in combination, a variable reactor including a coil, said reactor being variable in response to deflections of saidmember, a source of alternating potential for energizing said coil, said coil having a lagging current output which varies non-linearly with deections of said member, means in parallel circuit relation with said coil for providing a circuit having a leading power factor whereby a leading non-linear current is obtained, and circuit means having a predetermined value of impedance for correcting the non-linearity of said current comprising a full-wave rectifier, a filter for iiltering the output lof said rectifier and a current measuring instrument supplied. by said lter.

1l. Apparatus of the character set forth in claim l0 and in addition circuit means whereby a direct current bias is applied to the output of said rectifier for fixing the minimum value of current in said rectifier and eliminating the nearzero-current non-linear characteristics thereof4 12. A system for indicating the deflections of a -member subject to a stress comprising, in combination, a bridge circuit, a source of alternating potential for energizing said bridge circuit, a

reactor including a coil the inductance of which` isl variable inl response to deflections of said member associated with one leg of said bridge circuit, said coilhaving a lagging current output which varies non-'linearly` withl deilections of said member, means in parallel circuit relation with said coil for introducing a circuit characteristic whereby a leading non-linear current is obtained, a second coil having an adjustable inductance and a lagging current output associated with the leg of said bridge circuit adjacent that associated with the coll of said reactor, means in `parallel circuit relation with said second coil for introducing a circuit characteristic whereby a leadingcurrent output is obtained, said second mentioned coil being adapted to substantially balance said bridge circuit for zero deilection oi' said member, and current measuring means assopotential for energizing said bridge circuit, a ref-v actor including a coil the inductance of which is variable in -response to deflections of said member associated `with-one leg of said bridge circuit, said coil having a lagging current output which varies non-linearly with deflections of said member, means in parallel circuit relation 4with said coal for producing a circuit characteristic such that a leading non-linear current is obtained, a second coil having an adjustable inductance and a lagging current output associated with the leg of said bridge circuit adjacent that associated with the coil of said reactor, means in parallel circuit relation with said second coil for producing a circuit characteristic such thata leading current output is obtained, said second coil being adapte'dto substantially balance said bridge cir cuit for zero deflection of said member, and circuit means having a predetermined value of impedance for correcting the non-linearity of said current outputA of said first mentioned coil comprising a full-wave rectifier supplied by the unbalanced current of said bridge circuit, a filter for ltering the output of said rectifier and a current measuring instrument supplied by said filter.

14. A system for indicating the deflections of a member subject to a stress comprising, in com-4 blnation, a bridge circuit, a source of 7alternating potential for` energizing said bridge circuit, a reactor including a coil the inductance of which is variable in response to deflections of said member associated with one leg of said bridge circuit, said coil having a lagging current output which varies non-linearly with deflections of said memsociated with the leg of said bridge circuit adjacent that associated with the coil of said reactor,

means in parallel circuit relation with said second coil for producing a circuit having a leading power factor whereby a leading current output is obtained, said second coil being adapted to substantially balance said bridge circuit for zero deflection of said member, and circuit means having a predetermined value of impedance for correcting the non-linearity of said current output of said iirst mentioned coil comprising a fullwave rectier, with a series capacitor supplied by the unbalanced current of said bridge circuit, a lter for filtering the output of said rectier and a current measuring instrument supplied by said lter.

15. Apparatus c the character set forthin4 claim 13 and in addition circuit means whereby a direct current bias is applied to the output of said rectifier for xing the minimum Value of current in said rectifier and eliminating the near zero current non-linear current characteristic. THOMAS E. BROWNE, JR. 

